The subject matter disclosed herein relates generally to muffling systems, and, more specifically, to muffling devices capable of inducing high pressure drops and desirable flow properties.
In a gas turbine engine, air is pressurized in a compression module during operation. The air channeled through the compression module is mixed with fuel in a combustor and ignited, generating hot combustion gases which flow through turbine stages that extract energy therefrom for powering the fan and compressor rotors and generate engine thrust to propel an aircraft in flight or to power a load, such as an electrical generator.
In some gas turbine engines, a portion of the high-pressure air, such as, for example, bleed air from a compressor, may be extracted or bled from the compressor for various needs. These needs include, for example, compressor flow bleeding which may be used in order to improve operability as well as to provide turbine cooling, pressurize bearing sumps, purge air or provide aircraft environment control. The air may be bled off from the compressor using bleed slots located over specific portions or stages of the compressor.
The problem: In least some gas turbine engines, during engine operation occurring in some operating conditions, the compressor may pump more air than is required for needs including the combustion process. In order to manage operability of the engine and combustion performance, a portion of the excess bleed air from the compressor may be routed through bleed conduits and exhausted into the by-pass flow stream, engine exhaust, or to ambient. The pressure and temperature of the air stream bled from the compressor may be very high. For example, bleed air pressure may be greater than about 1375 kPa and the bleed air temperature may be greater than about 538 degrees C. A transient bleed valve system (TBV) system is sometimes used for bleeding and exhausting the air removed from the compressor. Certain conventional designs for bleed exhaust systems use large and/or heavy muffling devices to reduce the generated noise. For example, the exhaust area of some conventional bleed systems may be set to lower the flow velocity at the exhaust location to a level below that required to meet the acoustic limits for the application. The exhaust area, as well as the relatively gently expansions between the source pressure and exhaust, may contribute to the relatively large size and/or weight of these systems. In some applications (e.g., aircraft), it may be undesirable to use large and/or heavy components.
In addition, some conventional exhaust designs on aircraft may require extensive thermal shielding on other components near the exhaust location, once the exhaust velocities that meet the acoustic limits are achieved. Due to the nature of the high temperature air, once it is over-expanded to achieve lower velocities, the air it mixes with may overwhelm the bleed air, causing it to “lay down” on the surrounding structure around the engine. In some aircraft the surrounding structure may be made of lightweight composite material, or of other metallic material with lesser temperature capability.